An exception is a signal that a condition has occurred that can’t be easily
handled using the normal flow-of-control of a Python program. Exceptions
are often defined as being “errors” but this is not always the case. All
errors in Python are dealt with using exceptions, but not all
exceptions are errors.

To explain what an exception does, let’s review the normal “flow of control”
in a Python program. In normal operation Python executes statements sequentially,
one after the other. For three constructs, if-statements, loops and function
invocations, this sequential execution is interrupted.

For if-statements, only one of several statement blocks is executed and
then flow-of-control jumps to the first statement after the if-statement.

For loops, when the end of the loop is reached, flow-of-control jumps back
to the start of the loop and a test is used to determine if the loop needs
to execute again. If the loop is finished, flow-of-control jumps to the
first statement after the loop.

For function invocations, flow-of-control jumps to the first statement in
the called function, the function is executed, and the flow-of-control
jumps back to the next statement after the function call.

Do you see the pattern? If the flow-of-control is not purely sequential, it
always executes the first statement immediately following the altered
flow-of-control. That is why we can say that Python flow-of-control is
sequential. But there are cases where this sequential flow-of-control does
not work well. An example will best explain this.

Let’s suppose that a program contains complex logic that is appropriately
subdivided into functions. The program is running and it currently is executing
function D, which was called by function C, which was called by function B,
which was called by function A, which was called from the main function. This
is illustrated by the following simplistic code example:

defmain()A()defA():B()defB():C()defC():D()defD()# processing

Function D determines that the current processing won’t work for some reason
and needs to send a message to the main function to try something different.
However, all that function D can do using normal flow-of-control is to return
a value to function C. So function D returns a special value to function C
that means “try something else”. Function C has to recognize this value,
quit its processing, and return the special value to function B. And so forth
and so on. It would be very helpful if function D could communicate directly
with the main function (or functions A and B) without sending a special value
through the intermediate calling functions. Well, that is exactly what an
exception does. An exception is a message to any function currently on the
executing program’s “run-time-stack”. (The “run-time-stack” is what keeps track
of the active function calls while a program is executing.)

In Python, your create an exception message using the raise command. The
simplest format for a raise command is the keyword raise followed by
the name of an exception. For example:

raiseExceptionName

So what happens to an exception message after it is created? The normal
flow-of-control of a Python program is interrupted and Python starts looking
for any code in its run-time-stack that is interested in dealing with the
message. It always searches from its current location at the bottom of the
run-time-stack, up the stack, in the order the functions were originally
called. A try:except: block is used to say “hey,
I can deal with that message.” The first try:except: block that Python
finds on its search back up the run-time-stack will be executed. If there
is no try:except: block found, the program “crashes” and prints its
run-time-stack to the console.

Let’s take a look at several code examples to illustrate this process. If
function D had a try:except: block around the code that raised a
MyException message, then the flow-of-control would be passed to the
local except block. That is, function D would handle it’s own issues.

Let’s summarize our discussion. An exception is a message that something
“out-of-the-ordinary” has happened and the normal flow-of-control needs to
be abandoned. When an exception is raised, Python searches its run-time-stack
for a try:except: block that can appropriately deal with the condition.
The first try:except: block that knows how to deal with the issue is
executed and then flow-of-control is returned to its normal sequential execution.
If no appropriate try:except: block is found, the program “crashes” and
prints its run-time-stack to the console.

As our final example, here is a program that crashes because no valid
try:except: block was found to process the MyException message.
Notice that the try:except: block in the main function only knows how
to deal with ZeroDivisonError messages, not MyException messages.